For decades, paleoanthropologists have posited that hominins, the ancestors of modern humans, evolved gradually over millions of years. However, new research from the University of Reading significantly complicates this narrative. The most notable increase in body size took place approximately 2 to 2.5 million years ago with species such as Homo rudolfensis and Homo erectus/ergaster. In contrast, Homo floresiensis and Homo naledi maintained smaller statures akin to early humans like Australopithecus, averaging 40 kg in weight and resembling the height of a child. Other branches of Homo exhibited a marked increase in size, with Homo erectus/ergaster being the first to commonly exceed 60 kg, aligning their weight more closely with that of modern humans.

Body size is a critical aspect of organismal biology, influencing various factors including ecology, life history, physiology, and neuroanatomy.

In hominin evolution, increased body size is correlated with larger brain capacity, enhanced bipedalism, and a greater range of mobility.

Despite this significance, a consensus on constant increases in hominin body size over time remains elusive.

Recent studies provide conflicting perspectives, with some challenging the general evolutionary trend while others highlight increases in overall weight and height.

The smaller-bodied species, Homo floresiensis and Homo naledi, are frequently cited as exceptions.

Moreover, some researchers propose that increases in size might be lineage-specific, occurring within certain Homo groups.

However, lineage-specific size changes have yet to be analyzed alongside general trends using a unified model.

“Over time, various studies have drawn different conclusions about whether our ancestors consistently increased in size or experienced rapid growth during pivotal moments in human evolution,” stated lead author Dr. Jacob Gardner, a researcher at the University of Reading.

“The disparity arises because each study has focused on different parts of a much larger puzzle.”

“By integrating all available fossils, considering multiple competing theories, and clarifying inter-species relationships, a clearer picture begins to emerge. The answer likely incorporates elements from these varying theories.”

“The narrative of human evolution is not solely one of continuous growth; it involves significant changes that occurred later within our own genus, alongside the development of other lineages that took distinct evolutionary paths.”

In their recent study, Dr. Gardner and colleagues assessed estimated body weights from 386 fossil specimens across 21 human species, ranging from ancient australopiths to modern Homo sapiens.

Employing advanced statistical models to elucidate evolutionary relationships between species, they discovered that the dynamics of human body size are far more intricate than a straightforward upward trajectory.

The most prominent trend identified indicates a considerable increase in size, particularly in the later members of the Homo genus—occurring after Homo habilis—rather than a consistent growth through all of history.

Species such as Homo ergaster and Homo erectus exemplified a notable size jump, coinciding with other key evolutionary advancements, including improved bipedalism and dietary shifts towards carnivory.

A widespread gradual increase in size across all hominins receives moderate backing and is heavily influenced by the fossil datasets and estimation approaches applied, which remain contentious areas within the field.

Two species notably stand out as consistent anomalies: Homo floresiensis from Indonesia and Homo naledi, both of which consistently fall below the average weight forecasts for their respective time frames.

“Our findings indicate that the evolution of human body size isn’t merely a tale of steady progress,” remarked co-author Dr. Thomas Pushell from the University of Oxford.

“While there has generally been an increase in weight throughout our evolutionary timeline, the majority of significant shifts occurred later within the Homo genus.”

“This transformation aligns with broader developments in how our ancestors navigated their environments and adapted to varying ecological niches, underscoring the close relationship between body size and major behavioral and ecological changes.”

For more insights, refer to the findings published today in Proceedings of the National Academy of Sciences.

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Jacob D. Gardner et al. 2026. Competing models of human body size evolution. PNAS 123 (27): e2521732123; doi: 10.1073/pnas.2521732123